To develop solid oxide duel cells operating at temperatures lower than 700℃, stability and electrical property of various materials at high temperatures under various gas atmospheres were investigated. As new electrolyte materials, Ba_2In_2O_5 and La_<1-x>Sr_xGa_<1-y>Mg_yO_<3-δ> were estimated. The property of La_<0.6>Sr_<0.4>Co_<1-y>Ga_yO_<3-δ> and La_<1-x>Ca_xCrO_<3-δ> were analyzed from the viewpoint of cathode and interconnector materials, respectively.It was revealed that the phase of Ba_2In_2O_5 at temperatures more than 1070℃, which was discovered in this work, had high thermodynamic stability and oxide ion conductivity under oxygen partial pressure of 1〜10^<-4> atm ; however, it was not suitable for electrolyte materials since it decomposed at 900℃ under fuel gas atmosphere with oxygen partial pressures lower than 10^<-17> atm. On the contrary, La_<1-x>Sr_xGa_<1-y>Mg_yO_<3-δ> has high potential for new electalyte materials since it showed high thermal stability and oxide ion co
… Morenductivity under oxygen partial pressure of 1〜10^<-22> atm. It was also discovered that the structural phase transition of La_<1-x>Sr_xGa_<1-y>Mg_yO_<3-δ>, which is one of the problems for application, can be suppressed by partial substitution of Al for Ga site.La_<0.6>Sr_<0.4>Co_<1-y>Ga_yO_<3-δ> showed high oxide ion and hole conductivity under atmospheres for cathode electrode with oxygen partial pressure of 1〜10^<-4> atm. Since thermal expansion of La_<0.6>Sr_<0.4>Co_<1-y>Ga_yO_<3-δ> showed close behavior to those of electrolyte materials, it was estimated that La_<0.6>Sr_<0.4>Co_<1-y>Ga_yO_<3-δ> had high potential for new cathode materials. Prototype fuel cell employing La_<1-x>Sr_xGa_<1-y>Mg_yO_<3-δ>, La_<0.6>Sr_<0.4>Co_<1-y>Ga_yO_<3-δ> and Ni as electrolyte, cathode and anode, respectively, was developed and its property was estimated. We have succeeded in operation of the prototype cell at 600℃.We also analyzed property of La_<1-x>Ca_xCrO_<3-δ> as interconnectior under various gas atmospheres and revealed that the generation of oxide ion vacancy at temperatures more than 700℃ under reduction atmosphere caused the reduction expansion, which could be thermodynamically explained by ideal solution model. It was concluded that increase of electrical conductivity at less than 600℃ was necessary for application of La_<1-x>Ca_xCrO_<3-δ> as an interconector. Less